The present invention relates to a nondestructive method of and apparatus for ultrasonically detecting flaws which are produced with time in solid objects such for example as steel tubes manufactured by a centrifugal casting process.
Heating tubes for use in reforming a gas in contact with water vapor comprise heat-resistant cast tubes made of austenite steel such as HK-40 (including 0.40% of carbon, 25% of chromium, and 20% of nickel) and manufactured primarily by centrifugal casting for increased resistance against corrosion, heat, and pressure. A plurality of such cast tubes are welded together to provide a prescribed tube length. In use, steam and a raw material gas are fed under pressure into the tube filled with a catalyst, and the tube is heated to a high temperature by an external heater. Since the tube is subject to high temperature and pressure, it tends to develop, in about three years, time-dependent fissures caused by thermal stresses due to the temperature difference between the heated outer surface and the cool inner surface of the tube or defects in the inner surface of the tube due to such temperature difference. It is highly important for stable operation and safety of the tube to detect such time-dependent flaws in a nondestructive manner for accurately grasping the remaining service life of the tube.
Nondestructive testing methods for checking flaws in tubes manufactured by centrifugal casting include radiographic inspection and ultrasonic flaw detection.
The radiographic inspection method is effective to detect flaws which have certain dimensions in the direction in which the radiation is transmitted. The flaws can effectively be detected if they have a thickness of 1% or more of the wall thickness of an object being inspected and extend in a direction normal to the direction of travel of the radiation. Other defects such as cracks cannot be detected by the radiographic method. Therefore, the radiographic inspection process is low in its detecting accuracy.
The ultrasonic flaw detection is highly effective in inspecting internal flaws in relatively homogenenous steel materials such as general wrought steel and cast carbon steel. Various methods are known for ultrasonic flaw detection. For example, they include a reflection method, a through transmission method, and a resonance method. Some ultrasonic flaw detecting processes employ one or two ultrasonic probes, or emit an ultrasonic wave perpendicularly or obliquely to an object to be tested. These different inspection methods are selected dependent on the shape of the object, the type of flaws in the object, and other factors.
With the heat-resistant tube made of austenite steel by centrifugal casting, however, difficulty has been experiented in detecting any flaws therein since the tube material causes the ultrasonic energy to be dampened therein to a large extent and the displayed image is rendered complex by many echo pulses as large crystals cause grain boundary reflection. Another problem is that it is difficult to transmit a required amount of ultrasonic energy through the tube because of the casting surface on the outer tube surface. Therefore, the ultrasonic flaw detecting method has not extensively been relied upon for detecting defects in heat-resistant tubes of austenite cast steel since the ultrasonic flaw detection has a low rate of detecting defects in such tubes.
Japanese unexamined Patent Publication No. Sho 58-47252 discloses a technique for detecting flaws in a circumferential welded tube portion by measuring an ultrasonic energy having transmitted through the welded seam and opposite tube portions adjacent to the welded seam, and adding and substracting the three measured values according to evaluation equations to ascertain whether there is a defect or not. This flaw detecting technique however still has a low detecting accuracy since the material properties differ in the measured points and such property differences affect the noise level, and the three measuring points are not always selected with good accuracy. Japanese unexamined Patent Publication No. Sho 59-99251 shows the use of three ultrasonic probes which however suffer from the same drawbacks as mentioned above.
The applicant has filed Japanese Patent Application No. 59-146488 on a method of compensating for differences in the attenuation coefficient of the material. The disclosed method is however incapable of removing the problem of dispersion of the ultrasonic energy due to varying transmission and reception efficiencies of individual probes, different patterns (spreading patterns) of ultrasonic beams, and different material properties in beam paths.